1. Field of the Invention
The present invention relates generally to sealing devices, and more particularly to gaskets such as gaskets for use in gasoline and diesel engines, compressors, oil coolers, and other machinery.
2. Description of the Related Art
Gaskets have long been used to seal interfaces between components in a wide variety of machines and especially in gasoline and diesel engines. For example, head gaskets seal between the heads of an engine and the engine block, oil pan gaskets seal the interface between the oil pan and the block, and water pump gaskets seal around the ports of a water pump where the water pump is attached to the engine block. Most gaskets are specifically designed for their particular intended use. For instance, head gaskets are designed to seal against the high pressures and temperatures and the generally caustic environment within the cylinders of an engine. On the other hand, water pump gaskets must seal against leakage of coolant, which may consist of a water and anti-freeze mixture that is heated and under pressure. Many if not most automotive gaskets traditionally have been made of a compressible fibrous gasket sheet material that is die-cut to the required gasket shape.
In general, two key performance characteristics required of most compressible gaskets include compression failure resistance and sealability. Compression failure resistance refers to the ability of a gasket to withstand high compression forces when clamped between two flange surfaces without crushing, deforming, or yielding to the point that the mechanical properties of the gasket material and ultimately the seal provided by the gasket are compromised. Sealability refers to a gasket""s ability to resist or prevent leakage of fluid both between the gasket faces and the flanges between which the gasket is clamped (hereinafter referred to as interfacial leakage) and through the gasket material itself (hereinafter referred to as intersticial leakage).
Leakage can be of particular concern with compressible fibrous gaskets, which generally are fabricated from sheets of material composed of fiber, filler, and a binder. Because of their fibrous nature and because apertures of the gasket typically are die-cut, the gasket edges surrounding the apertures tend to be somewhat porous. Since these porous edges usually are exposed to the fluid being sealed, intersticial leakage can be a particular problem with fibrous gaskets. Interfacial leakage can be caused by compression failure of the gasket material or by rough or warped flange surfaces. Thin flanges and poor bolt placement can result in regions of substantially reduced compression stress on a gasket, which also can lead to interfacial leakage.
In some instances, the sealability of a gasket can be enhanced by providing all of the surfaces of the gasket with a coating or by impregnating the gasket with a resin. Fibrous gaskets are particularly likely to have such treatments since, in many cases, the porous material of the gasket itself, although compression failure resistant, is subject to intersticial and interfacial leakage as a result of the failure mechanisms discussed above. U.S. Pat. No. 3,661,401 discloses a gasket having a coating that covers both the exposed gasket faces and the edges that surround and define various internal apertures of the gasket. U.S. Pat. No. 4,499,135 discloses a fibrous gasket that is impregnated with a silicone resin to improve its resistance to leakage of water-antifreeze mixtures. Similarly, U.S. Pat. No. 4,600,201 discloses a gasket impregnated with a polymerizable liquid impregnating agent to enhance sealability.
While coating and impregnation can improve the sealability of a gasket, unfortunately they inherently tend to degrade the compression failure resistance of the gasket. This is because, among other things, the coating and impregnating agents, which themselves exhibit good sealability but poor compression failure resistance, tend to penetrate and become a part of the gasket material. This reduces the gasket""s overall compression failure resistance and thus reduces the ability of the gasket to function well under higher flange pressures where compression failure is more likely. As a result, coated and impregnated gaskets such as those disclosed in U.S. Pat. Nos. 3,661,401, 4,499,135 and 4,600,201 can perform poorly under high flange pressures, which severely limits the applications in which such gaskets can be used.
Other gaskets include special fillers to enhance their sealability. For example, U.S. Pat. No. 5,240,766 discloses a soft porous gasket sheet material formed from fiber a binder, and a filler that provides enhanced sealability at higher temperatures. U.S. Pat. Nos. 5,536,565 and 5,437,767 also describe a gasket sheet material formed from fiber and a gel-forming mineral filler that provides the gasket with enhanced sealing properties, especially against polar liquids. While such fillers, like coatings and impregnations, can improve the sealability of gaskets, they also tend inherently to degrade the compression failure resistance of the gasket material and therefore reduce the ability of the gasket to withstand higher flange pressures. As a result, gaskets with specialized fillers to enhance sealability such as those disclosed in U.S. Pat. Nos. 5,240,766, 5,536,565 and 5,437,767 also can be severely limited in range of application.
It will thus be appreciated that for fibrous and perhaps other types of compressible gaskets, sealability and compression failure resistance have heretofore been mutually incompatible gasket properties. In other words, measures taken to enhance the sealability of such gaskets inherently tend to reduce compression failure resistance and vice versa. As a result, manufacturers of gaskets, and particularly fibrous gaskets, have engaged in proverbial balancing acts in order to design and produce gaskets with acceptable sealability and also acceptable compression failure resistance for a particular application. The problem, of course, is that each of these properties necessarily becomes a compromise and neither is optimized.
Another type of gasket used in many applications is known as a controlled compression rubber gasket. These types of gaskets incorporate molded rubber or polymer beads that are placed into a flanged joint in such a way that the amount of compression or compressive stress applied to the bead is predetermined and fixed by incompressible members. Such gaskets can take several forms. One form of a controlled compression rubber gasket is the common O-ring gasket, wherein a molded rubber bead is nested in a groove formed in the mating surface of one of a pair of flanges. The depth and width of the groove are carefully determined such that the compression stress on the rubber when the flanges are bolted together is known and thus controlled. In another form of controlled compression rubber gasket, a rubber bead or strip is molded onto the interior edge of a metal or plastic shim or carrier surrounding an interior aperture. The rubber bead is wider than the thickness of the shim and therefore can never be compressed to a thickness smaller then the thickness of the shim when the gasket is clamped between a pair of mating surfaces. Thus, the amount of compression applied to the rubber bead is limited by the thickness of the shim. In another example, a rubber bead is molded into grooves on one or both sides of a plastic carrier, which is disposed in a joint to be sealed. Metallic compression limiters, such as washers embedded in the carrier or shouldered bolts, provide a positive compression limit on the rubber and plastic of the gasket. Controlled compression rubber gaskets may also be found in the form of a rubber sheet or coating of a specific shape and profile molded onto both sides of a metal carrier with embedded washers or other means of compression limitation used to control the amount of compressive stress applied to the rubber coating.
U.S. Pat. No. 5,194,696 of Read illustrates one type of controlled compression rubber gasket wherein a rubber bead is molded onto the interior edge of a incompressible plastic carrier, the bead being wider than the thickness of the carrier. The gasket is placed between the mating flanges of a hard disc drive case and the flanges are bolted together until they engage the plastic carrier. The rubber bead is thus compressed between the flanges but never less than the thickness of the carrier such that the compressive stress applied to the bead is limited by the carrier thickness.
While the physical form of controlled compression gaskets varies, the sealing mechanism is common to all. Specifically, the beads of such gaskets are formed from a polymeric or rubber compound that is reasonably stable when in contact with heat and the particular fluid being contained. The spring rate of the compound in conjunction with the limited maximum compression stress provided by the carrier thickness or other compression limitation mechanism and the stiffness of the flanges yield a predetermined minimum and maximum surface stress between the rubber bead and the flange surfaces sufficient to prevent interfacial leakage. Spring rate of the bead is determined by the type and degree-of-cure of the rubber or polymer compound, the shape and contact area of the bead, and the thickness of the bead. The thickness of the compression limiter or depth of the groove in the case of O-ring seals is carefully designed to yield a compression stress on the bead that is sufficient to form a seal but not so high as to crush the bead. It will thus be seen that the performance of controlled compression gaskets is highly dependent upon the characteristics of the bead material and degree of compression provided by the compression limiting components. Too much compression can lead to crushing of the bead while too little can result in insufficient compression stress to establish a seal.
While controlled compression rubber gaskets have been used in many applications, they nevertheless suffer from a failure mechanism known as Compressive Stress Relaxation (CSR) failure in which the surface stress that prevents interfacial leakage diminishes over time. The CSR failure mechanism is a combination of several competing effects including, but not limited to, rearrangement of polymer molecule chains in response to the stress state, shrinkage of the bead due to molecular chain cross-linking, softening and swelling of the bead due to fluid penetration, and degradation of the polymer molecule chains due to heat, fluid, and oxygen exposure. Since the flange gap in which the bead resides is fixed by rigid compression limiters, these competing effects tend to reduce the compressive stress on the bead over time, which leads to leakage. Further, controlled compression gaskets tend to be substantially more expensive to manufacture than die-cut fibrous gaskets, which, among other factors, makes controlled compression gaskets an unacceptable alternative to fibrous gaskets in many applications.
A need therefore exists for an improved compressible fibrous gasket that retains the economy and wide application range of traditional fibrous gaskets and that also provides a superior and longer lasting seal. The properties of sealability and compression failure resistance should be de-coupled such that each can be optimized for a particular application without compromising the other. Such a gasket should exhibit excellent to complete sealability in a wide variety of joint types while at the same time having the highest possible resistance to compression failure where such failure is likely. The failure modes associated with controlled compression rubber gaskets should be successfully addressed, as should problems with warped or rough flange surfaces. A method of fabricating such a gasket that is economical, efficient, and reliable is also needed. It is to the provision of such a gasket and fabrication method that the present invention is primarily directed.
Briefly described, the present invention, in a preferred embodiment thereof, comprises an improved compressible fibrous gasket that exhibits simultaneously both excellent sealability in a wide range of joints and outstanding compression failure resistance. The gasket comprises a base sheet of substantially planar contiguous fibrous gasket material having a predetermined thickness and two opposed substantially parallel faces. The term xe2x80x9ccontiguousxe2x80x9d as used herein means that the base sheet is uninterrupted across its flange width; that is, the gasket material of the base sheet extends continuously across the base sheet without breaks or innerlineations. This includes layered gaskets such as rubber coated metal gaskets wherein the layers are contiguous as defined herein. In general, the term xe2x80x9cgasket materialxe2x80x9d as used herein when referring to the invention includes any appropriate porous and/or layered material or both, but is not intended to include rigid carriers such as the carriers of controlled compression rubber gaskets. Such carriers provide mechanical support and compression limitation for their rubber seals, but generally do not contribute to the gasketing or sealing functions of the gasket. The term xe2x80x9cbase sheetxe2x80x9d when used alone without being identified as a base sheet of gasket material is intended to include rigid carriers and all other gasket materials.
The gasket material of the invention can be any of a number of traditional gasket sheet materials, but most preferably is a fibrous gasket material formed of a fiber and a binder and perhaps a filler. The base sheet has a flange width across its faces and is configured to define at least one interior aperture bounded by a substantially porous interior edge of the base sheet. In many instances, the aperture and interior edge are formed by a die-cutting process, which reveals the porous internal structure of the gasket material on the interior edge. A substantially porous exterior edge of the gasket extends around and defines the outside periphery of the base sheet.
An edge coating, which preferably is a polymeric coating, but that can be formulated of a latex or other suitable material, is disposed on the porous interior edge of the base sheet surrounding the gasket aperture. The material of the edge coating at least partially penetrates the exposed pores on the edge of the base sheet forming a relatively narrow intrusion zone surrounding the aperture. This intrusion zone seals the porous edge, anchors the edge coating to the base sheet, and densities the material of the base sheet in the region immediately surrounding the aperture to concentrate available compressive stress in this region when the gasket is clamped between mating surfaces. The coating itself is formulated and configured to engage, conform to the shape of, and adhere to the mating surfaces to establish a significantly enhanced seal as compared to traditional fibrous gaskets.
The edge coating can take on any one of a variety of physical configurations according to the particular intended use of the gasket. In one and perhaps the most preferred embodiment, the edge coating is wider than the thickness of the base sheet so that the edge coating projects beyond the facial planes of the base sheet to define projecting rims that extend around the aperture of the gasket. In another embodiment, a relatively narrow face coating is provided on one or both faces of base sheet extending in a strip around the aperture. The face coatings may be formed of a different material than that of the edge coating with the face coating abutting the edge coating around the gasket aperture. Preferably, however, the edge coating surrounding the aperture is applied in such a way that it wraps around onto the faces of the base sheet to form the face coatings, in which case the edge and face coatings are made of the same material. In either event, it is important at least in regions of high compression stress to limit the width and thickness of the face coating strips as detailed below to minimize their detrimental effect on the compression failure resistance of the gasket.
A unique method of fabricating gaskets according to the present invention is also provided. Briefly described, the method, referred to herein as a xe2x80x9cstack-and-coatxe2x80x9d process, comprises stacking a predetermined number of cut gasket base sheets together with their apertures aligned with each other. The aligned apertures form a cavity having the outer contours of the aperture and a depth determined by the number of gaskets in the stack. According to one preferred methodology, the base sheets are stacked atop a plate having a shallow well formed therein, the well having a shape corresponding to the shape of the gasket aperture and being aligned with the apertures of the stacked gaskets. Coating material, such as a polymer, in liquid form is placed in the well and the cavity is closed off. The entire assembly is then tilted on edge and rotated at a predetermined relatively slow rate and through a predetermined number of revolutions. During rotation, the liquid polymer flows around the perimeter of the cavity and contacts the exposed edges of the stacked base sheets.
As the polymer flows around the perimeter of the cavity over and over again, it gradually builds up on the edges of the base sheets to form a coating on the walls of the cavity with a portion of the polymer penetrating into the porous gasket material of the edges to form intrusion zones. When a sufficient number of revolutions have been completed to build up a coating of a desired thickness, the assembly is tilted back down to allow excess polymer to drain back into the shallow well of the plate, whereupon the stack can be removed.
After allowing the polymer coating to thicken partially but not completely, the individual gaskets are peeled off of the stack.1 Since the polymer is only partially thickened and thus still malleable, the peeling of each gasket causes the polymer on the gasket""s edge to stretch and deform rather like soft taffy, which results in an edge coating that projects beyond the facial planes of the gasket to form the opposed projecting rims. The edge coatings are then fully thickened in an oven or otherwise to set the final shape and physical properties of the edge coating.
The terms xe2x80x9cthickenxe2x80x9d, xe2x80x9cthickenedxe2x80x9d, and terms of similar import are used herein to refer to the gradual transformation of the coating from its more liquid initial form to its more solid final form. xe2x80x9cPartially thickenedxe2x80x9d means that the coating is in a state between the two forms in which it retains a measure of malleability. Thickening can occur through a variety of physical and chemical mechanisms including curing (the cross-linking of polymer chains within the coating material) and drying (the evaporation of solvents from the coating material). All such mechanisms are intended to be encompassed within the meaning of the term xe2x80x9cthickenedxe2x80x9d as used herein. 
In an alternative methodology referred to herein as a xe2x80x9cmold-in-placexe2x80x9d process, base sheets of gasket material are stacked with their apertures aligned as above but with one or more spacers disposed between the base sheets. The walls of the cavity formed by the stack are coated as described. The spacers have apertures that can be slightly smaller or slightly larger than the apertures of the base sheets. If a spacer with a slightly larger aperture is disposed between each base sheet, a narrow gap is formed between each sheet and polymer flows a slight distance onto the faces of each base sheet to form overlapped face coatings surrounding the apertures of the gaskets. Spacers with larger apertures produce edge coatings that do not project beyond the facial planes of the gasket. A precisely molded wrapped edge coating can be formed by stacking a larger aperture, then a smaller aperture, then another larger aperture spacer between each of the base sheets of gasket material. In either event, edge and face coatings are formed on the gaskets.
Alternative methodologies for coating the interior edges of the stacked base sheets are also envisioned and form part of the invention. These alternative methodologies include a xe2x80x9cstack-and-fillxe2x80x9d process wherein the base sheets are stacked and the cavity formed by their aligned apertures is filled with a polymeric coating material. After a predetermined time, the coating material is drained or poured out of the cavity, leaving a coating on the interior edges of the gaskets. Other methodologies include a xe2x80x9cstack-and-sprayxe2x80x9d process wherein the coating material is sprayed onto the interior edges of the stacked base sheets, and a xe2x80x9cstack-and-wipexe2x80x9d process wherein the coating material is wiped or spread onto the interior edges with a squeegee or other appropriate tool. These and other methodologies are encompassed by the stack-and-coat process of the present invention.
Edge coated compressible gaskets according to the present invention provide outstanding sealability and eliminate the failure modes of traditional gaskets in at least the following ways. Application of a polymer edge to a compressible base sheet yields a complex sealing mechanism that maximizes tolerance to flange surface imperfections (roughness, warping, and deflection) and creates a tight fluid seal with a minimum of clamp load. This is accomplished through selection of a relatively soft conformable polymer for the edge coating that, when applied to form protruding rims relative to the faces of the base sheet, is highly conformable to flange surface imperfections. As the polymer edge is compressed to near the thickness of the base sheet, the attachment of the polymer edge to the base sheet provides a significant stiffening effect, which dramatically increases the spring rate of the edge in compression. This allows significant sealing force to be generated in the polymer edge while using a soft conformable polymer that is able to accommodate significant compression strain.
Further, the intrusion zone created by migration of the polymer into the edge pores of the base sheet creates a band of higher density around the gasket aperture, which serves to concentrate compressive load where it is most needed to enhance the seal.
Additional factors also contribute to the outstanding performance of gaskets of this invention. These include the use of a polymer that is impervious to the fluid to be sealed, which prevents intersticial leakage. The conforming of the edge coating to flange surface imperfections, the development of sealing stress through compression of the edge coating and the intrusion zone, and the selection of polymers that develop surface adhesion to the flange surfaces all contribute to an outstanding seal against interfacial leakage. The combination of the sealing mechanisms of compression stress and surface adhesion results in a seal that, over time, is more tolerant to degradation of either or both. For instance, in the event that compression stress of the polymer edge coating drops over time to a level below that needed to create an initial seal given the flange condition, fluid type, and fluid pressure, a leak still will not occur because an adhesive bond has developed between the material of the edge coating and the flange surfaces.
Application of a polymer edge coating to a compressible base sheet according to the present invention also successfully addresses the problem of compression stress relaxation failure common in controlled compression rubber gaskets. Specifically, the compressible nature of the base sheet material results in a natural thinning of the base sheet over time due to compression stress. This thinning causes the flange surfaces to move slightly closer together over time, which actually increases the compressive stress on the polymer edge coating. This increase in compressive stress, which cannot occur with controlled compression gaskets, usually is more than sufficient to offset any stress relaxation that may be experienced by the polymer edge coating.
Embodiments of the present invention with face coating strips address compression stress relaxation through an additional mechanism. Compression stress on the bead of a traditional controlled compression gasket usually ranges from about 100 to 1000 pounds per square inch (psi). Compression stress relaxation can cause a loss of from 60 to near 100 percent of the initial sealing stress on the bead, often resulting in insufficient compression stress to maintain a seal. However, in the face coated embodiments of the present invention, initial sealing stress on the coating material can range from 1,000 to 10,000 psi. Provided that a polymeric material is selected that can accommodate such levels of stress, a loss of even 90 percent of the initial sealing stress still does not reduce remaining sealing stress below the level necessary to maintain the seal. Thus, stress relaxation failure modes of traditional prior art gaskets are virtually eliminated by edge coated gaskets of the present invention.
It will be appreciated from the forgoing that a unique and improved gasket is now provided that addresses and solves the long-standing problems with prior art gaskets. The gasket of the present invention, because of its uniquely configured high sealability edge coating, provides a seal around the aperture of the gasket that is outstanding and, in many cases, near perfect. At the same time, since the edge coating provides such an exceptional seal, the base sheet or flange portion of the gasket requires little or no coating or impregnation to enhance its sealability. As a result, the maximum compression failure resistance providable by the fibrous gasket material of the base sheet is preserved.
Traditional failure modes of the edge coating itself, such as stress relaxation failure, are also virtually eliminated through the complex sealing mechanisms and edge coating configurations of the invention. The ultimate result is a highly reliable long lasting gasket that exhibits exceptional sealability and outstanding compression failure resistance simultaneously. In addition, the edge coating material itself can be specifically formulated for the particular use to be made of the gasket. For example, water pump gaskets can be provided with an edge coating that is particularly resistant to water/anti-freeze mixtures whereas the edge coatings on oil cooler gaskets can be formulated to seal against petroleum based oils. Finally, the physical configuration of the edge coating can be tailored for the particular joint type to be sealed. For instance, thicker wider edge coatings may be called for where the gasket is to be used with rough or warped flange surfaces or with thin flanges where compression stress can vary greatly due to flange deformation. On the other hand, thin narrow edge coatings may be chosen to seal flat smooth flange surfaces or highly stressed joints.
While the combination of excellent sealability and preserved compression failure resistance is a particularly advantageous property of the present invention, it will be appreciated that the edge coating of the invention provides unique advantages independent of compression failure resistance. For instance, compression failure resistance is not always a concern or a design specification, especially when sealing joints that are not highly stressed. In these situations, the edge coated gasket of this invention still provides enhanced sealability independent of whether or not compression failure resistance is preserved. Thus, the invention should not be deemed to be limited to the combination of these features, although each may be present in many of the preferred embodiments disclosed herein. These and many other features, objects, and advantages of the gasket and method of the present invention will become more apparent upon review of the detailed description set forth below when taken in conjunction with the accompanying drawing figures, which are briefly described as follows.